1. Field of the Invention
The invention relates to a converter, and more particularly to a quasi-resonant half-bridge converter and a control method thereof.
2. Description of the Related Art
Conventional direct current voltage converters may have some of the following drawbacks:
1. Poor conversion efficiency at light load. Conventional SRCs (series resonant converters) have primary side switches operating at ZVS (zero voltage switching), to thereby have higher conversion efficiency. However, since power of the load of the SRC is associated with a switching frequency of the primary side switches, the primary side switches may require a higher switching frequency at light load, so that the magnetic components and switch components of the converter may have a greater switching loss, resulting in poor conversion efficiency of the converter at light load.
2. Narrow range of operating power. Since conventional flyback quasi-resonant converters operate in one-quadrantal magnetization, a larger transformer may be required for promoting power range. In addition, since the power switches and the output capacitor thereof operate in BCM (boundary-conduction mode), higher stress and greater capacitance may be required.
3. Complicated switch control. Since conventional flyback quasi-resonant converters control conduction of switches by detecting voltages across the switches, or by detecting an end of power transmission of the transformer, the converters operate in BCM. Control circuits may need to have driving signals with variable-frequency control due to detection of relatively low points of the voltages across the switches, resulting in complicated control.
4. Difficult EMI (electromagnetic interference) design. Since conventional SRCs and quasi-resonant converters use variable-frequency switch control to generate a stable DC (direct current) output voltage, the operating frequency of the circuit may vary within a range, so that all of the frequency bands within the range should be considered into EMI design.
5. Poor ripple performance. Conventional flyback quasi-resonant converters operate in one-quadrantal magnetization, thereby requiring larger output capacitance for reducing output voltage ripple compared to those that operate in bi-quadrantal magnetization.
6. Larger size. When a conventional flyback quasi-resonant converter is applied to a load of greater than 60 watts, the transformer and the output capacitance required for the converter may be larger to fulfil requirements of circuit design due to greater output currents and ripples.
7. Components capable of withstanding higher voltages may be required. Since conventional clamping converters have a nature of a voltage across the switch being a sum of the output voltage and reflection of the output voltage to the primary side according to a turn ratio of the transformer, components capable of withstanding higher voltages may be required.
8. Low utilization rate. Conventional quasi-resonant converters primarily operate in one-quadrantal magnetization, thereby having a lower transformer utilization rate compared to those that operate in bi-quadrantal magnetization, and a larger transformer may be required to complete the circuit design.